Hollow airfoils used as helicopter rotor blades, steering rotors, wings, elevators, rudders, and the like, have been made of a variety of different materials employing various methods of manufacture. Such airfoils have been made of metal such as aluminum, and, increasingly, have been manufactured of composite fiber materials, such as glass, plastic, molded carbon filament, and the like. To produce hollow airfoils of such composite fiber materials, relatively complicated multi-step manufacturing processes are used.
For example, U.S. Pat. No. 4,169,749 discloses a method for making a hollow aircraft rotor blade from laminated material. An expandable mandrel first is constructed from curable materials. A composite build up of epoxy materials then is made by layering a plurality of pre-preg plies of curable material over the expandable mandrel. The mandrel and epoxy build up materials then are placed in a mold to define the configuration of the outer surface of the airfoil. Pressure is applied to expand the mandrel, which in turn expands the build up of material against the mold. The mandrel and composite build up of material then are cured with the mandrel becoming an integral part of the rotor blade.
Another approach is disclosed in U.S. Pat. No. 3,967,996. In this patent, a hollow airfoil is manufactured by forming a blank of single sheets with an adhesive substance applied to them. The sheets are stacked onto a mandrel and placed in a mold. Preliminary compression is employed to provide the blank with a stiffness and shape in an unclosed profile. The blank ends are brought together to create a cavity, which accommodates an elastic bag. The blank with the bag in it is placed in a mold, and air pressure is fed into the elastic bag to perform a final pressing of the sheets onto the internal surfaces of the mold.
The U.S. Pat. No. 2,773,272 discloses a method of making a laminated airfoil structure. The airfoil of this patent is of a non-uniform (that is, tapered) cross section. A precut plastic core first is compressed in the mold, around a longitudinal center spar. The mold is opened and the core pieces are removed. The upper and lower skins for the airfoil then are placed on opposite sides of the mold, and the precut plastic core and spar are placed between them. Everything then is cured together; so that the airfoil itself is not hollow, but has a plastic core in it.
The U.S. Pat. No. 3,962,506 is directed to a complex multi-step procedure for forming helicopter blades. Inflatable inserts are used in each of the different curing steps to press the parts against the mold; so that the final result is a structure with internally formed longitudinal supports or spars. The multi-chambered blade, however, is not formed in a single curing operation, but requires multiple curing steps in order to provide the hollow blade with internal supports.
The U.S. Pat. No. 4,565,595 is directed to a method of making a composite aircraft wing. This patent is similar in many respects to U.S. Pat. No. 3,962,506. Several separate inflatable mandrels are stacked together in the process of the '595 patent. The covering layers then are placed over these various mandrels. The assembly is placed in a mold; the mandrels are inflated; and the wing is cured. The mandrels then are deflated and removed from the final structure.
Another U.S. patent directed to a method for producing a hollow helicopter rotor blade is U.S. Pat. No. 5,248,242. The rotor blade of this patent is manufactured with a variety of separate pieces laid up together to form a core inside the blade. This includes a pre-cured bridge, along with a machined core. Wrapping around a mandrel forming the forward part of the blade functions to create a longitudinal reinforcing spar in the finished product. Along with the placement of solid core members, which remain in the blade after it is cured, an inflatable rubber bag also is included. The bag is inflated during the curing process. This is a complex, multiple step process.
It is desirable to provide a method for manufacturing a hollow, one-piece, molded composite airfoil with integral reinforcing members in it, which overcomes the disadvantages of the prior art listed above, and which is simple and effective.